An optical transceiver module is an optical communications device used to transmit and receive optical data signals over optical waveguides (e.g., optical fibers) of an optical communications network. An optical transceiver module includes a transmitter (TX) portion and a receiver (RX) portion. The RX portion of an optical transceiver module typically includes one or more receive photodiodes for detecting optical data signals received over one or more optical fibers and for producing corresponding electrical signals. Electrical circuitry of the RX portion detects and processes the electrical signals produced by the receive photodiodes to recover the data contained in the received optical data signals.
The TX portion of an optical transceiver module includes input circuitry, a laser driver circuit, one or more laser diodes, and an optics system. The input circuitry typically includes buffers and amplifiers for conditioning an input data signal, which is then provided to the laser driver circuit. The laser driver circuit receives the conditioned input data signal and produces electrical modulation and bias current signals, which are provided to the laser diodes to cause them to produce optical data signals having logic 1 and logic 0 intensity levels corresponding to the electrical bits contained in the input data signal. The optical data signals are then directed by the optics system of the TX portion onto the ends of respective transmit optical fibers held within a connector that mates with the optical transceiver module.
The TX portion of an optical transceiver module typically also includes an open loop or closed loop optical output power control system that monitors and controls the modulation and/or bias currents of the laser diodes in such a way that the average optical output power levels of the laser diodes are maintained at substantially constant levels. Open loop optical output power control systems do not directly measure the optical output power levels of the laser diodes, but rather, rely on temperature, age and/or other parameters to determine adjustments that need to be made to the bias and/or modulation currents of the laser diodes to maintain their average optical output power levels at substantially constant average optical output power levels. Closed loop optical output power control systems use monitor photodiodes in the TX portion to monitor the output power levels of the laser diodes and feedback circuitry to produce control signals that are then used to adjust the modulation and/or bias currents of the laser diodes such that their average optical output power levels are maintained at substantially constant levels. Closed loop optical output power control systems are generally more accurate than open loop optical output power control systems due to the fact that closed loop systems react in real time based on real time measurements to make the necessary adjustments to the modulation and/or bias currents of the laser diodes.
FIG. 1 illustrates a block diagram of a typical TX portion 21 of an optical transmitter or transceiver module that includes a closed loop optical output power control system. The TX portion 21 includes a buffer 31, a pre-drive amplifier 32, a laser driver circuit 33, and a laser diode 34. The TX portion 21 typically also includes an optics system (not shown) for directing the light produced by the laser diode 34 onto the end of a transmit optical fiber (not shown). For ease of illustration, the optics system of the TX portion 21 is not shown in FIG. 1. The buffer 31 receives an input data signal at its input and adds some gain to the input data signal. The pre-drive amplifier 32 adds some additional gain to the input data signal and provides an output signal to the laser driver circuit 33. The laser driver circuit 33 provides a modulation current and a bias current to the laser diode 34 based on the amplified input data signal output from the pre-drive amplifier 33 that cause the laser diode 34 to produce optical output signals having logic 0 and logic 1 power levels that represent the logic 0 and logic 1 electrical bits, respectively, contained in the input data signal.
The closed loop optical output power control system of the TX portion 21 comprises a optical output power feedback control loop made up of a monitor photodiode 22, a transimpedance amplifier (TIA) 23, a low pass filter (LPF) 24, a power monitoring circuit (PMC) 25, an analog-to-digital converter (ADC) 26, and a controller device 27. The monitor photodiode 22 detects the optical data signals produced by the laser diode 34 and produces corresponding electrical current signals. The TIA 23 detects the electrical current signals produced by the photodiode 22 and produces an output voltage signal, which is integrated by the LPF 24 to produce an average voltage level. The PMC 25 receives the average voltage level produced by the LPF 24 and outputs an analog voltage level value indicative of the average optical output power level of the laser diode 34. The analog power level value is input to the ADC 26, which converts the analog value into a digital value and provides the digital value to the controller device 27.
The controller device 27 is configured to perform various algorithms to control the TX portion 21. One of these algorithms uses the digital value representing the average optical output power level of the laser diode 34 to produce one or more laser control signals, which are delivered to the laser driver circuit 33. The laser control signals are adjusted by the controller device 27 based on the digital value corresponding to the detected average optical output power level value received by the controller device 27 from the ADC 26. These adjustments cause the laser driver circuit 33 to adjust the bias and/or modulation currents of the laser diode 34 such that the average optical output power level of the laser diode 34 is maintained at a substantially constant level. The controller device 27 also produces an Enable signal that can be used to enable/disable the laser diode 34 based on a control algorithm, external signal, or fault monitoring circuits (not shown).
The optical output power monitoring feedback loop of the TX portion 21 has a low cutoff frequency due to the frequency response of the LPF 24. This low cutoff frequency limits the frequency content that the input data signal to the TX portion 21 can have. For example, if the input data signal comprises a long string of consecutive logic 1s or logic 0s, this frequency of such a pattern may be below cutoff frequency of the feedback loop, resulting in the feedback loop causing improper adjustments to be made to the modulation and/or bias currents of the laser diode 34. Consequently, the feedback loop will not be effective at maintaining a constant average optical output power level for the laser diode 34 when the frequency content of the input data signal drops below the low frequency cutoff of the feedback loop.
One way to extend the lengths of the strings of consecutive logic 1s or logic 0s that can be handled by the TX portion 21 is to lower the cutoff frequency of the feedback loop. The typical method to reduce the low frequency cutoff of the feedback loop involves using larger circuit elements in the feedback loop circuitry. However, such solutions have the undesired impact of larger circuit elements that tend to increase die area and costs.
Another problem associated with attempting to lower the cutoff frequency of the feedback loop is that doing so increases the link startup time period of the TX portion 21. The LPF 24 of the feedback loop integrates the output of the TIA 23 to obtain an average value. The TX startup time period is generally determined by the amount of time that is required for the LPF 24 to converge to its steady state. Lowering the cutoff frequency of the LPF 24 increases the amount of time that is required for the LPF 24 to converge to its steady state. Industry or customer constraints place an upper limit on how long the link startup time period can be, which, in turn, places a lower limit on the cutoff frequency of the LPF 24.
Accordingly, a need exists for an optical TX having an optical output power control system and method that provide accurate results when the optical TX is transmitting relatively long patterns of consecutive logic 1s or consecutive logic 0s. A need also exists for such an optical output power control system that provides these advantages without increasing the startup settling time period of the optical TX.